If you have a theory, you have to test your theory, and you must fisrt have a hypothosis, then thest your hypothosis with an expirement, then draw a conclusion.
In your case you theorise your cat is bad, and you can test this by looking inside. Experiment: drop exhaust after cat and look inside, if cat is bad replace, if cat is good, move to next theory.
Now, if your cas is bad, you have to find out why it went bad. Unless a cat is manufactured defective, the only way for it to malfunction is with somthing wrong upstream in the exhaust.
